FIG. 12 shows the arrangement of the permanent magnets of a linear motor according to one prior art device. FIG. 12 shows only one of two permanent magnet groups opposing each other through a holding member 7. Permanent magnets 110a and 110b alternately arrayed on the holding member 7 have a non-rectangular parallelepiped shape with corners being cut off. The permanent magnets 110a and 110b are arrayed such that their magnetisms (N and S poles) are alternately opposite. Also, the thicknesses and widths of the permanent magnets 110a and 110b are so adjusted as to generate an ideal sine wave magnetic field in a space between opposing yokes and the permanent magnet groups.
According to the above prior art arrangement, to make the sine wave magnetic field generate an ideal sine wave, the thicknesses and widths of the permanent magnets must locally differ. As the entire shape of each magnet is not a simple rectangular parallelepiped, each permanent magnet is difficult to fabricate with high precision, leading to a high cost. This problem has not been solved yet. The obtained magnetic field density is lower than that obtained with a rectangular parallelepiped permanent magnet, and the driving force of the linear motor is accordingly decreased.